A hair follicle drug test works by analyzing a sample of hair for traces of drugs. Drugs enter the bloodstream when a person uses them, and hair growth is fed by the bloodstream. As a result, drug molecules are deposited in the hair shafts, where they can be detected by laboratory testing. Most hair follicle drug tests can detect cocaine, marijuana, opiates, amphetamines and phencyclidine.
Most of the time, only the 1.5 inches of hair closest to the scalp are analyzed in a hair follicle drug test. Hair grows at the rate of about half an inch per month, so testing reveals whether any drugs have been used in approximately the last 90 days. If a person does not have 1.5 inches of hair on their head, hair samples may be taken from elsewhere on the body. Some laboratories claim that drugs can be detected in body hair up to a year after use, though no conclusive evidence for this assertion exists.
Hair follicle drug tests are more expensive and time-consuming to administer than urine tests, but they provide a longer window into a person's history of drug use and are much more difficult to cheat than urine tests are. Some people claim that certain chemicals or shampoos can fool a drug test, but there is no evidence that any external agents can alter the internal chemical makeup of hair.